How does one get the options associated with a protocol buffer field?
E.g., suppose I have a field with a custom options:
message Foo {
optional string title = 1 [(indexed) = true];
}
I can get a list of fields:
for f in foo.ListFields():
print f
How do I access the "indexed" state? (I can see there is a list of f "_options", but that seems "internal"? Is there a proper way to access option extensions by name)?
I'll use as an example the nanopb custom options, as defined here. However the answer itself is not in any way nanopb-specific, nanopb uses the standard protobuf style for custom options:
message NanoPBOptions {
optional int32 max_size = 1;
...
}
extend google.protobuf.FieldOptions {
optional NanoPBOptions nanopb = 1010;
}
and an option defined like this:
message Person {
optional string email = 3 [(nanopb).max_size = 40];
}
The API used to get the option value varies between languages. However the basic flow is the same:
In Python:
desc = person_pb2.Person.DESCRIPTOR
field_desc = desc.fields_by_name['email']
options = field_desc.GetOptions()
value = options.Extensions[nanopb_pb2.nanopb].max_size
In Java:
desc = PersonProto.Person.getDescriptor();
field_desc = desc.findFieldByName("email");
options = field_desc.getOptions();
value = options.getExtension(Nanopb.nanopb).getMaxSize();
In C++:
desc = Person::descriptor()
field_desc = desc->FindFieldByName("email");
options = field_desc->options();
value = options.GetExtension(nanopb).max_size());
nanopb_pb2.py. Is there any way of doing this on the fly (i.e. oh I need these custom options, let me compile their proto so I can access them)? Secondly, when I access field_desc.options.Extensions[nanopb_pb2.nanopb].max_size I get 0. Any ideas as to what I may be doing wrong?
import nanopb_pb2) as generated by the default plugin before calling CodeGeneratorRequest.ParseFromString, but how is the plugin going to know what it needs to import before calling CodeGeneratorRequest.ParseFromString?